1. Field of the Invention
The present invention relates to digital color printing in which neutral grey objects and color objects are intermixedly printed, and more particularly, in which the composition of colorants of the neutral color for each neutral objects is automatically changed depending on the presence or absence of background color which contains non-black colorants.
2. Description of the Related Art
Digital color printers use a plurality of colorants, such as cyan, magenta, yellow, and black, to form images which appear to the human visual system to contain a multitude of colors. In particular, a neutral color may be formed in a number of ways. First, it may be formed of equal or nearly equal combinations of cyan, magenta, and yellow. In practice, the black colorant is also often added to the combination of cyan, magenta, and yellow colorants. When a neutral color is formed with a mixture of some of cyan, magenta, yellow, and black, it is a type of neutral known as process neutral. Second, a neutral color may also be formed by only using the black colorant. This type of neutral color is known as single-color neutral.
Each type of neutral, process neutral and single-color neutral, have advantages and disadvantages when used in different contexts in the printing process.
When single-color neutral is used over a color field, it is often the case that the single-color neutral colorant has a gloss which is different from the gloss of the surrounding color field, and the gloss difference can be objectionable. Also, the ink coverage or toner pile height of single-color neutral may be noticeably less than that of the surrounding color background. Finally, the presence of misregistration between the plurality of separations during the printing process, or the presence of other interactions between the separations, can result in objectionable white or light-colored fringing around a single-color neutral object on a color background. This occurs for example if the single-color neutral colorant is not printed precisely and without distortion into the hole left at that position when printing the other separations.
On the other hand, when process neutral is used in a white or grey field which does not contain any non-black colorants, slight misregistrations or interactions among the plurality of separations can result in objectionable color fringing at the edges of the object. Furthermore, the use of multiple colorants to form process neutral can result in blurred edges, thickened objects, and objectionable amounts of ink or toner coverage.
These advantages and disadvantages of single-color and process neutral are known in the graphics arts industry, and knowledgable graphic artists attempt to design pages which utilize the advantages and avoid the disadvantages. However, some situations, such as a neutral object placed only partially over a color field, or a color background which changes rapidly, or complex text or graphics over a complex color background, makes it difficult to manually avoid the printing problems associated with mixedly printing neutral colors and other colors. Furthermore, many pages not designed by a knowledgeable graphic artist can benefit from a method and apparatus to automatically create a background dependent neutral image.